This invention relates to digital monitoring techniques. More particularly, the invention relates to a modified statistical parity calculation technique affording significant bit rate reduction through data compression without the usual requirement of deterministically altering a pre-existing bit stream.
A useful testing function in digital data communications is the ability to verify that two data streams are identical when they contain the same information while only differing in phase or absolute timing. Such a test readily confirms proper operation of digital transmission equipment. Conventionally, a bit by bit comparison technique was used wherein a variable length digital delay is appropriately adjusted to obtain phase synchronization of the signals before application to an exclusive OR gate. The output of the exclusive OR gate then indicates discrepancies or bit errors between the two digital bit stream signals.
Increases in the capacity of sophisticated digital communications systems by utilization of higher bit rates has rendered conventional error monitoring techniques much more difficult in a number of ways. Namely, the requirements of the variable delay are made more difficult since it must operate at the same high speed clock rate as the data signal. The capacity of the variable delay must also be able to compensate for the delay experienced in the system under test.
It often is required to include the transmission medium as a necessary component of the portion of the system under test. In a typical transmission system many miles of transmission medium must be included. For example, 500 miles of transmission cable is not unusual. At a data rate of 139 MBS, the variable delay required for monitoring of such a transmission system must accommodate 10,000 bits or time slots. The combination of these constraints severely limits the speed and size of the system capable of being tested by conventional error monitoring techniques.